The present invention relates to a flicker reducing circuit, and more particularly to a flicker reducing circuit in a terminal that displays character data, image data, and the like on a display unit. The present invention also relates to a flicker reducing method.
FIG. 13 is a block diagram illustrating a terminal having a conventional flicker reducing circuit. This terminal consists of a data processing unit 80, an interactive unit 81, a line control unit 82, a display processing unit 83, a vertical low-pass filter 84, delay units 85 and 86, an exchange switch 87, a memory unit 88, an interlace converting unit 89, an NTSC encoder 90, and a display unit 91. The data processing unit 83 is connected to the interactive unit 81, the line control unit 82, and the display processing unit 83 via a bus 93. The data processing unit 81 outputs character data and graphic data, or character data and composite data in which character data is combined with graphic data, input to the interactive unit 81.
The interactive unit 81 receives character data and graphic data in an interactive mode. The line control unit 82 controls a connection between a telephone line 92 and the bus 93 to which the data processing unit 80, the interactive unit 81 and the display processing unit 83 are connected. The display processing unit 83 stores character data and composite data output from the data processing unit 80 to the memory unit 88 and reads character data and composite data out of the memory unit 88 to display them. The vertical low-pass filter 84 passes low-frequency components among spatial frequencies in vertical direction of a screen of composite data processed in the display processing unit 83.
The delay unit 85 delays composite data processed in the display processing unit 83 by a process time in the vertical low-pass filter 84 and then outputs the result. The delay unit 86 receives character image data formed of only character data among data processed in the display processing unit 83 and delays it by a process time in the vertical low-pass filter 84, then outputting the result. The changeover switch 87 selects the output data from the vertical low-pass filter 84 or the output data from the delay unit 85 according to the output signal from the delay unit 86.
The interlace converter 89 converts composite data selectively output from the changeover switch 87 into data to be displayed in an interlace mode. The NTSC encoder 90 converts the composite data converted in the interlace converter 89 into a composite signal conformed to the NTSC system. The display unit 91 displays the composite data in a composite signal mode extracted by the NTSC encoder 90.
Next, the operation of the conventional flicker reducing circuit will be explained below. First, the interactive unit 81 input character data and graphic data encoded in a predetermined encoding system to the data processing unit 80 via the line control unit 82. The data processing unit 80 creates composite data by combining the character data with the graphic data input from the interactive unit 81, and then inputs the composite data to the display processing unit 83, together with the character data.
The character data and the composite data input to the display processing unit 83 is stored into the memory unit 88. The memory unit 88 includes a composite image line memory for storing the composite data and a character image line memory for storing character data. The composite data is stored into or read from the composite image line memory while the character data is stored into or read from the character image line memory.
The display processing unit 83 reads out the composite data stored in the composite image line memory within the memory unit 88, and then outputs it as image data for a non-interlace operation which sequentially scan a screen from its upper portion toward its lower portion, to the vertical low-pass filter 84 and the delay unit 85. The character data stored in the character line memory within the memory unit 88 is input to the delay unit 86. The vertical low-pass filter 84 is a filter which passes low-frequency components of less than a predetermined frequency of spatial frequency components in the vertical direction on an image.
Regarding the character data, the data processing unit 80 detects a character displaying position at the time a character frame has been received, and then combines the character information frame showing the position as a set of dots. That is, character data output from the display processing unit 83 to the delay unit 86 becomes image data representing the presence or absence of a flag.
The composite data input to the delay unit 85 and the character data input to the delay unit 86 are delayed by the process time of the vertical low-pass filter 84. The character data output from the delay unit 86 is supplied as a switching signal to the changeover switch 87. Thus, the changeover switch 87 selects the low-frequency component of the composite data passing through the vertical low-pass filter 84 for a period during which a flag "1" is set to the character data output from the delay unit 86 (during a period for a character display position). The delay unit 85 also selects output composite data for a period during which the flag "1" is not set.
That is, the changeover switch 87 selects the low-frequency component of composite data passing through the vertical low-pass filter 84 at a character display position and selects the output composite data from the delay unit 85 which do not pass through the vertical low-pass filter 84 at positions other than character display positions.
The composite data selected by the changeover switch 87 is image data displayed in a non-interlace display system. However, since the method of displaying composite data on the display screen of the display unit 91 is a 2:1 interlace displaying system in which a first field and a second filed are alternately scanned, the interlace converter 89 converts composite data into data in the 1:2 interlace display system.
The NTSC encoder 90 receives the composite data output from the interlace converter 89. In the NTSC encoder 89, a color signal of the composite data is balanced-modulated. The composite data is converted into an NTSC system composed composite signal band-shared and duplexed in the high-frequency area of a brightness signal. The converted signal is input to the display unit 91. Thereafter, the display unit 91 color-displays the composite data from the NTSC encoder 90 converted into a composite signal on the screen.
In the display unit 91 which displays in the 2:1 interlace displaying system, since each scanning line is refreshed every frame period (e.g. 30 seconds), as well known, flickering occurs at each bright spot on a scanning line. In actual, neighboring scanning lines are overlapped each other. In the NTSC television system, two neighboring scanning lines are set to have such a frequency relation that when the brightness of a scanning line in one field is minimized, the brightness of a scanning line in the other field is maximized. Hence, the brightness of the two scanning lines is interpolated so that flickering does not occur clearly.
When an image is formed of one horizontal line, bright spots appear only on one scanning line, but there are no bright spots on the neighboring scanning lines. In this case, occurrence of flicker is conspicuous. Particularly, one horizontal line is seen in many characters. In many case, characters are not interrelated with neighboring scanning lines. Hence, occurrence of flickers is conspicuous in characters.
In the flicker reducing circuit of the terminal shown in FIG. 13, in order to display character data and graphic data in a superimposition mode, data passing the vertical low-pass filter 84 are selected in character data area while data output from the display processing unit 83 being input data of the vertical low-pass filter 84 are selected in areas other than character data area. Thus, images are displayed at positions other than character positions without removing high-frequency components of a spatial frequency in a vertical direction. Flicker is removed by removing high-frequency components of a spatial frequency in a vertical direction in a character image at a character position.
The conventional flicker reducing circuit reduces only the flicker of the character image at a character position. However, this flick phenomenon occurs at positions other than character display positions. When the noticeable flicker occurring at a character display position is reduced, a flicker becomes unexpectedly prominent even at graphic data display positions where flicker is not conspicuous. For that reason, in order to reduce the flicker, it is needed to detect not only character display positions but also one horizontal line displayed with graphic data. In this case, it is required to execute a low-pass filtering to a narrow band because a flicker of one horizontal line in graphic data image and the like is not noticeable compared with that at a character display position.
Conspicuous flicker can be removed at character display positions. However, if an ideal low-pass filter (never obtainable in actual) is not used that cuts only frequency components of a predetermined frequency or more, blurring occurs on the boundary portion. In actual, correction is needed to add the originally-included signal component removed by the low-pass filter.